DESCRIPTION (Verbatim from the applicant's abstract) The success of improved graft survival over the last decade has heightened the awareness of the long-term complication of classical immunosuppression, hence focusing future research on the development on tolerogenic strategies. It has become apparent that peptides play a central role in determining T cell responses to alloantigen. This has lead to an increasing interest in the potential use of synthetic peptides to manipulate T cell responses to foreign antigen. Peptides derived from both polymorphic and non-polymorphic regions of the MHC have been shown to significantly impact allograft survival in animals models, and clinical trials using non-polymorphic MHC peptides in humans are currently underway. Our preliminary data demonstrates that non-polymorphic MHC class II derived peptides inhibit the proliferative response to autoantigen, and alloantigen presented by both direct and indirect pathways. These immunomodulatory effects are mediated through the deletion of antigen presenting cells and T cell unresponsiveness. We hypothesize that the inhibitory peptides mediate their effects through binding to MHC class II, disrupting the interaction of the TCR with the MHC+peptide complex and thereby modulating the immune response. The aims of the research proposal are to investigate the mechanism of action mediating the immunomodulatory effects of these peptides and to determine their role in preventing allograft rejection. We will define the pathways leading to the induction of apoptosis in antigen presenting cells and determine the relative susceptibility of the different professional antigen presenting cells to deletion. T cell signaling patterns in unresponsiveness T cells will be investigated, and studies performed to determine whether the lack of T cell response to subsequent stimulation is mediated by anergy, deletion or immune deviation. The sequence specific nature of inhibition by the peptides will be evaluated by amino acid substitutions. Binding studies will performed to determine both the site of binding, and the relative binding affinity of the original and altered peptides. The ability of non-polymorphic MHC class II peptides to prolong allograft survival and induce tolerance will be evaluated in a mouse cardiac transplant model. Gene transfer of peptide constructs to cardiac allografts will be performed to investigate the benefits of local versus systemic delivery on graft survival. The mechanisms mediating long-term allograft survival will be investigated in vitro and in vivo. Peptides will be combined with other immunomodulators to develop novel tolerogenic strategies. These studies will help elucidate the potential role of MHC class II peptides in the prevention of transplant rejection and the induction of tolerance.